This invention relates to the production of electrically conductive polymer materials and is particularly concerned with the production of such materials exhibiting improved mechanical properties, processability, and thermal and environmental stability, and with procedure for producing same.
The free-base form of polyaniline is believed to comprise subunits having the formula: ##STR1## where n is between 0 and 1. The oxidation state of polyaniline referred to as "emeraldine" is believed to have a value of n of about 0.5.
This free-base form of polyaniline is an electrical insulator. Reaction of emeraldine free-base with protonic acids of the form HX, where X is, for example, Cl, causes the polymer to undergo an insulator to conductor transition, as disclosed in A. G. MacDiarmid, et al, Mol. Cryst. Liq. Cryst. 121, 173 (1985). Conductive polyaniline of this type has been employed in batteries as disclosed, for example, in French Patent No. 1,519,729.
However, a number of difficulties have been encountered with the prior art materials noted above. Thus, the conductive polyaniline acid salts are, with a few exceptions, insoluble in most solvent media. None of the polyanilines can be melted. The emeraldine free-base and the conductive forms thereof noted above tend to form powders on removal of the solvent. With some effort, films can be cast; however, they are quite fragile and brittle, easily crumbling to form a powder. The conductive acid salts lose their conductivity when exposed to liquid water. This loss is due to deprotonation. The conductivity loss is reversible; treatment of othe deprotonated material with protic acids restores the conductivity. Further, conductive regions in an insulating matrix tend toward diffusion. For example, if one makes a conductive trace of polyaniline acid salt on a substrate of emeraldine free-base, the trace remains spatially stable for only a short time, eventually spreading out until the substrate has a constant conductivity throughout.
Some of these problems were addressed in U.S. Applications Serial No. 920,474 filed Oct. 20, 1986, now U.S. Pat. No. 4,798,685, of S. I. Yaniger, and Ser. No. 013,305 filed Feb. 11, 1987, now U.S. Pat. No. 4,806,271, of S.I. Yaniger, et al, both assigned to the same assignee as the present application. In these applications, it is disclosed that Lewis acids, for example, alkylating agents, can be used to make the insulating emeraldinen free-base into a conductive polymer salt. Use of proper Lewis acids resulted in conductive polyanilines with the Lewis acid as a side chain. These derivatized polyanilines are more water stable and processable than the prior art emeraldine acid salts. Additionally, no diffusion between "doped" conducting and "undoped" insulating regions was observed.
Thus, in the above U.S. application, Ser. No. 920,474, a base-type non-conductive polymer, such as polyaniline, can be reacted with, for example, methyl iodide, to form an electrically conductive polymer in which the methyl group is covalently linked to the nitrogen atoms of the polymer.
In the above U.S. application, Ser. No. 013,305, emeraldine free-base can be reacted with reagents of the form RSO.sub.2 C1, e.g., tosyl chloride, to form an electrically conductive polymer in which the -SO.sub.2 R groups are covalently linked to the nitrogen atoms of the polymer.
U. S. Application Ser. No. 158,477 filed Feb. 22, 1988, of S.I. Yaniger and R. E. Cameron and assigned to the same assignee as the prsent application, discloses reaction of a base-type non-conductive polymer, such as polyaniline, with an anhydride, such as tosylic anhydride or benzophenone tetracarboxylic dianhydride, and forming an electrically conductive polymer in which the --SO.sub.2 R and --COR groups are covalently linked to the nitrogen atoms of the conductive polymer.
In general, however, the conductive polymers of the above applications tend to be brittle, resulting in inferior mechanical properties.
It would be desirable to blend the relatively brittle conducting polymer with a flexible polymer to form a blend having both the desired electrical properties and good flexibility. A suitable polymer for blending is polyimide. Polyimides have, in general, good mechanical properties, flexibility and thermostability.
To achieve high electrical conductivity, the proportion of conductive polymer in the blend must be relatively high (e.g., greater than 50%) in order for charge to be transferred effectively between polymer chains. Unfortunately, at high polyaniline loadings, the blend materials tend to phase separate, that is, the polyaniline aggregates into clumps within the non-conductive polyimide matrix. These clumps are separated by the matrix material, and the blend thus is an insulator. Further, the mechanical properties of the material suffer upon phase separation. It would be desirable to form blends where the polyaniline is dispersed evenly on a molecular level at all loadings, to thus form a conductive polymer blend.
An object of the present invention accordingly is the provision of improved electrically conductive polymer materials of the class of conductive polyaniline blended with a polyimide.
Another object is to provide conductive polymer materials having improved flexibility, mechanical properties, and thermal stability in the form of a continuous phase blend of a conductive polymer, e.g., conductive polyaniline, and a polyimide.
A still further object is to render polymides conductive by doping with a conductive polymer, such as polyaniline, to produce an easily processable, highly thermally stable conductive polymer blend.
A still further object is to provide novel procedure for producing the above conductive polymer blends.